Fruitgrowers Dam

Fruitgrowers Dam, located on Alfalfa Run, is 3 miles north of Austin, Colorado. It is an earthfill, rock-faced structure, 55 feet high and 1,520 feet long, containing 136,000 cubic yards of material. The reservoir stores a total of 4,540 acre-feet of water. The spillway, located on the left side of the dam, is an uncontrolled structure (meaning flows aren't regulated). A 76-foot-long concrete-lined channel discharges into a stilling basin which slows the velocity and reduces the energy of the water. The outlet works consists of one 3-foot diameter pipe controlled by two slide gates.

Geology

Bedrock at Fruitgrowers Dam is Mancos Shale of Cretaceous age. The Mancos Shale is a dark gray marine formation and has a gentle northerly dip in the vicinity of the dam. The shale is brown, soft, and has abundant secondary gypsum present when weathered. There is no evidence of faulting in the foundation area of the dam.

On the left abutment, the shale is covered with a thin layer of clay slope wash. The topography of this abutment suggests the presence of land-slide or slump deposits. Two inclinometers have all shown slight movements within the abutment. These inclinometers have all shown slight movement in a northwest direction at rates of 0.2 to 0.4 inches per year (0.5 to 1.0 cm/year). These movements are occurring at depths of 20 to 40 feet (6 to 12 m).

In the valley bottom, stream alluvium was found to be up to 23 feet (7 m) thick over Mancos Shale. This alluvium is composed of clayey and gravelly soils and contains some basalt cobbles and boulders.

The right abutment is a gently sloping area with deep glacial outwash deposits over the shale bedrock. These deposits were found to be up to 155 feet thick and contain gravel, cobbles, and boulders of basalt derived from the lava flows on Grand Mesa.

Readings of observation wells installed in 1979 indicate the presence of two ground water systems at Fruitgrowers Dam. One body of ground water occurs in surficial materials, and the second is confined water under pressure in the underlying Mancos Shale.